This invention concerns a new method and apparatus for in-line measurements of moisture contents in wood pieces, logs, lumber, veneer and the like. The common method of moisture measurements is based on determining of the difference in the weight of the substance before and after heating it at the temperature above the boiling point of water (usually at 105.degree. C.). However, such method requires long periods of time (24 hours or longer). That method is limited mostly to stationary measurements and it is not practical for in-line applications where virtually instantaneous water content measurements are required.
Another method of moisture determination is based on spectral selective optical absorption which can be measured in scattered and (or) transmitted beams. Sets of light sources with precisely selected wavelengths are used in that method. One set of the wavelength(s) is utilized as a reference which is relatively unaffected by the characteristic (e.g. moisture content) being measured. The other set of the wavelength(s) is tuned to the water absorption peak. Several versions of this approach have been disclosed, in which either conventional infra-red optical sources (e.g. U.S. Pat. Nos. 4,266,878, 4,634,856, 4,788,853, 3,851,175, 3,965,356, 4,052,615, 4,300,049, 4,465,929, 4,577,104, 4,733,078, 4,823,008, 4,840,706, 4,879,471, 4,928,013) or a submillimeter laser (U.S. Pat. No. 4,755,678) were applied. These methods were disclosed for moisture measurements either in soil or in paper products.
Moisture measurements based on absorption of fast neutron rays and gamma radiation were disclosed in U.S. Pat. Nos. 4,766,319 and 3,748,473.
Electrical meters based on measurements of an electrical property of a substance are used for the moisture content measurements where a relationship between the electrical property and the moisture content of a substance can be established. The moisture content of a substance may be estimated from electrical properties such as resistance, capacitance and conductivity. However, these type of meters are limited to lower end of the moisture content [according to The Standard Test Methods (ASTM: D 4444-84) the moisture content range for the electrical meters is from 6% to 27%]. The electrical meters can not be used efficiently outside that range of the moisture content because higher moisture levels cause no substantial change in electrical properties.
A method of non-contact moisture measurements is disclosed in U.S. Pat. No. 4,612,802 issued Sep. 23, 1986 to Forintek Canada Corporation. In that method, an area of one surface of a substance is subjected to a predetermined intensity of a heat source for a selected period of time. The moisture content is determined from measurements of the temperature rise of the heated surface of the substance.